1. Field of the Invention
The present invention relates to a method and device for forming an image, and more particularly to a method and device that can effectively reduce and correct a change/variation in an image surface caused by an environmental temperature change.
2. Discussion of the Background
An optical scanning device is commonly known in relation to an image forming apparatus such as a digital copier, a laser printer, a facsimile, an optical plate-maker, an optical plotter, and other similar devices. It has become common to use a resin lens in a scanning imaging optical system because of cost and easiness of forming a lens surface into a special shape.
A variance of a radius of curvature and refractive index of a resin lens caused by a change in an environmental temperature is large as compared to a corresponding variance of a glass lens. Thus, if the environmental temperature changes, an image surface of an optical beam spot is shifted away from a scanned surface. Hence, a size of a diameter of the optical beam spot is increased, resulting in a degradation of a produced image due to a lowered resolution.
Recently, a multi-beam system was employed in the optical scanning device to increase the efficiency of a scanning operation. If a change in the image-surface position of the optical beam spot occurs in the multi-beam optical scanning device, a distance between scanning lines of optical beam spots adjacent to each other in a main scanning direction (i.e., a scanning pitch) differs from a standard predetermined distance. Thus, a degraded image is formed due to an increased optical beam spot diameter.
Therefore, if a resin optical element is employed in the optical scanning system and a good image quality is desired, then a change in an optical property of the resin optical element due to the change in the environmental temperature needs to be corrected. A method to perform the above correction is disclosed in Japanese Patent Laid-Open Publication No. 8-292388, Japanese Patent No. 2804647, and Japanese Patent No. 2736984.
In the method disclosed in the above background art, a shift of an image-surface position in an optical axis direction caused by the change in the environmental temperature is corrected. However, if an image-surface curvature occurs, an appropriate correction is not made.
In FIG. 5A, I and SF denote an image surface and a scanned surface, respectively. In more detail, the image surface corresponds to a space location where the optical light coming from the optical scanning system is designed to be focused and create a clear image while the scanned surface is defined as a physical surface of a photosensitive member. For a high quality image device, the image surface and the scanned surface must coincide. In the present invention, I is the image surface in the main scanning direction and SF is the scanned surface. For simplification, the image-surface curvature in the main scanning direction is ideally corrected in FIG. 5A. Thus, the image surface I in the main scanning direction matches the scanned surface SF.
In FIG. 5B, a position of the image surface I is changed due to the change in the environmental temperature and separated from the scanned surface SF. The position of the image surface I is shifted to a position that is parallel to the scanned surface SF. However, a shape of the image surface I is not deformed.
As illustrated in FIG. 5B, if the optical property of the optical element has been changed due to the temperature change, a light flux forms the image at position I and then becomes a diverging light flux that reaches the scanned surface SF and forms an optical beam spot thereon. FIG. 5C is a drawing illustrating an enlarged view of FIG. 5B. The light flux that forms the optical beam spot forms a beam waist in a vicinity of a geometric optical image forming point (i.e., an intersection of rays of light indicated by a solid line in FIG. 5C). A diameter of the beam spot in the vicinity of the beam waist changes as indicated by a dotted line in FIG. 5C. A high quantity image is obtained when a position of the beam waist matches the scanned surface SF. Thus, a designed diameter of the optical beam spot in the main scanning direction corresponds to the diameter of the beam waist d.
However, as illustrated in FIG. 5C, if the position of the image surface I is shifted relative to the scanned surface SF because the optical property of the optical system has changed due to the change in temperature, a diameter D of the optical beam spot formed on the scanned surface SF in the main scanning direction becomes larger than the designed diameter d of the optical beam spot, which is referred to as an increased beam diameter phenomenon. In FIG. 5C, the light flux is strongly converged in the main scanning direction to make the diameter of the optical beam spot small in the main scanning direction.
To the contrary, the diameter D of the optical beam spot in the main scanning direction is comparatively small in FIG. 5D. Thus, the light flux is not strongly converged in the main scanning direction. In this case, a size of the diameter D of the optical beam spot formed on the scanned surface SF does not largely differ from that of the designed diameter d of the optical beam spot.
In a relationship between the diameters D and d, a range in which an increased beam diameter will not cause a serious problem in the quality of the produced image is referred to D less than d+xcex94d, where xcex94d is the range in which the increased beam diameter does not degrade the quality of the image. A range of a distance on both sides of the beam waist position that satisfies the expression D less than d+xcex94d is referred to as xcex94L. The xcex94L is then referred to as a depth allowance. If a distance between the image surface I and the scanned surface SF is within the range xcex94L, the increased beam diameter phenomenon, which may have an adverse effect on the quality of the produced image, does not occur.
FIGS. 5C and 5D show that the depth allowance depends on a degree of convergence of the light flux. Because the light flux that forms the optical beam spot is strongly converged as the size of the diameter of the optical beam spot decreases, the range xcex94L becomes small, resulting in a narrow depth allowance.
If the position of the image surface I is shifted from the scanned surface SF and a shape of the image surface I does not substantially change from the shape thereof when the image surface I is not shifted as illustrated in FIG. 5B, a correction on the shift of the image surface I may not always be required when the diameter of the optical beam spot is relatively small because the shifted amount of the imaging surface I is maintained within the depth allowance as illustrated in FIG. 5D.
A method disclosed in the above-described background art is directed to perform a correction on the shift of the position of the image surface I when the shift does not involve the change in the shape of the imaging surface I. In other words, this method shifts the position of the imaging surface I on a certain linear direction but does not change the shape of the image surface I. If the image surface I was curved before the shifting, for example, the image surface I remains curved after the shifting and the curvature is not corrected.
When a resin imaging element is used, the shift of the position of the image surface I caused by the change in the environmental temperature involves also the change in the shape of the image surface I. FIG. 5E is a drawing in which the position of the image surface I is shifted from the scanned surface SF and the shape of the image surface I is deformed into an arc shape.
When the shape of the imaging surface I changes as illustrated in FIG. 5E, the diameter of the optical beam spot is increased so that it becomes larger in the central portion than in both ends portions, when a vertical direction in FIG. 5E represents the main scanning direction. In this instance, the change in the diameter of the optical beam spot involves a change in the image height and thereby a degradation of the image is noticeable.
When the position and shape of the image surface I change as illustrated in FIG. 5E, if a correction is made by the method disclosed in the above-described background art, the position of the image surface I is shifted as illustrated in FIG. 5F, for example. Thus, the diameter of the optical beam spot in the vicinity of an optical axis is adjusted to an appropriate size. However, the size of the diameter of the optical beam spot in both end portions of a writing region becomes large and the change in shape of the image surface I is not corrected, thus the quality of the image produced is degraded.
The shift of the position of the image surface in the main scanning direction was described above. The position of the image surface in the sub-scanning direction shifts in an identical manner. If a multi-beam scanning operation is performed when the temperature changes, then a change in the shape of the image surface is observed and the quality of a written image is noticeably degraded because the scanning pitch changes in addition to the change in the image height.
The present invention has been made in view of the above-mentioned and other problems and addresses the above-discussed and other problems.
The present invention advantageously provides a novel optical scanning device including a scanning imaging optical system using a resin optical element so that an occurrence of an image-surface curvature caused by a change in an environmental temperature is effectively reduced. The present invention further provides a novel optical scanning device employing a scanning imaging optical system using a resin imaging element wherein the occurrence of the image surface curvature and a shift of the image surface from a scanned surface caused by the change in an environmental temperature is effectively reduced and corrected, respectively.
According to an example of the present invention, a scanning imaging optical system includes a first optical system configured to receive a light flux emitted from a light source, a second optical system configured to receive the light flux from the first optical system and condense the light flux to form a long linear image in a main scanning direction in a vicinity of a deflecting surface of an optical deflector, and a third optical system configured to condense the light flux deflected by the optical deflector toward a scanned surface to form an optical beam spot on the scanned surface. The third optical system includes at least one resin imaging element so that a maximum value xcex94Mmax and a minimum value xcex94Mmin of an amount of change xcex94M in an image-surface curvature in the main scanning direction at each image height in an effective writing region with respect to a change xcex94T in an environmental temperature satisfy a condition: |(xcex94Mmaxxe2x88x92xcex94Mmin)/xcex94T| less than 0.01 (mm/xc2x0 C.).
According to another example of the present invention, the third optical system includes at least one resin imaging element so that a maximum value xcex94Smax and a minimum value xcex94Smin of an amount of change xcex94S in an image-surface curvature in a sub-scanning direction at each image height in an effective writing region with respect to a change xcex94T in an environmental temperature satisfy a condition of: |(xcex94Smaxxe2x88x92xcex94Smin)/xcex94T| less than 0.01 (mm/xc2x0 C.).